A variety of designs for hovercraft air-cushion vehicles have been proposed which rely on paired horizontal thrusters for propulsion. In one example, U.S. Pat. No. 3,265,142 to Winter, separate propeller means are provided within each thrust tube.
In the prior art, U.S. Pat. No. 3,605,937 to Kirwan discloses a hovercraft which is propelled in forward or reverse directions by a doubly-directed ducted flow of air. Air to propel an air-cushion supported vehicle is directed in either the forward or reverse directions by a pair of complementary front and rear flaps or panels, arranged to rotate within such opposite ends in an out-of-phase relationship.
The phase relationship of these flaps is such that all of the available air in the duct may be directed alternately in the forward or in the reverse directions, or may be proportioned between such directions. This is accomplished by a series of linkages that cause these flaps to rotate, in butterfly valve fashion, in unison at their respective locations. As one flap opens, the other flap closes. This allows air flow to be allocated to exit from the duct at either end in desired proportions.
A single fan is proposed in Kirwan to provide both lift for the vehicle and forward or reverse thrust. This fan pressurizes the air-cushion cavity or region between the vehicle and the ground; and pressurized air for the directed ducts is drawn directly from this region.
Kirwan relies on the opposed ends of the ducts to provide forward and reverse propulsion. To effect changes in direction, steering vanes, in the form of rudders, are placed in the path of the air exiting the propulsion ducts. These vanes are separately controlled through rotation of a steering wheel arrangement.
Kirwan mentions the possibility of providing two or more for ducts to provide a resultant horizontal thrust component. But he stipulates that such ducts must be arranged so that the resultant horizontal component of thrust cancels out in a steady state. He also states that this resultant thrust is preferably arranged so that the air directed from the ducts passes in a direction, along a line which passes through the vertical central axis of the vehicle.
The consequence of this last stipulation is that multiple ducts, as proposed by Kirwan, function only to provide linear propulsion. Steering in such an arrangement is effected through only the steering vanes.
Kirwan suffers from a deficiency that arises from the use of a single air-compression fan or propeller. This fan imparts a rotational momentum to the compressed air, and the reaction on the fan is transferred to the vehicle as an inclination or bias to counter-rotate in the opposite direction to the rotation of the fan blades. While this tendency may be overcome by vectored thrusts of corrective air jets emitted horizontally in the appropriate direction, the accurate balancing of this effect is difficult to control precisely.
An earlier invention by one of the co-inventors herein, U.S. Pat. No. 3,746,116 to John Schwingshandl, proposes to overcome this pivotal effect by dividing the main air cavity inside the air-cushion vehicle skirts into at least four separate compartments. Air to each compartment is then provided through butterfly valves from an intermediate chamber (described in Schwingshandl as "unseen") located between the air-cushion cavity and the fan. Through use of the butterfly valves, the flow of air into each compartment is controlled. As such, this air flow is variable and not pre-fixed, or determined by fixed vehicular arrangements.
Schwingshandl relies on the use of rudder blades placed in the wash from horizontal ducts for directional control, as well.
These prior art arrangements lack the convenience of the features of the invention, which will now be described.